1. Field of the Invention
The invention relates to an image forming apparatus such as a copier or a printer utilizing the electrophotographic process, and particularly to a color image forming apparatus for forming a color image comprising toner images of a plurality of colors superimposed one upon another.
2. Description of Related Art
Generally in an image forming apparatus of the electrophotographic type, a fluctuation occurs to the density characteristic of a printed image due to the environment of use, the fluctuations of the characteristics of a developing device and a photosensitive drum by the number of printed sheets, the unevenness of the sensitivity of the photosensitive drum during the manufacture thereof, the unevenness of the triboelectrification characteristic of a toner during the manufacture thereof, etc.
An effort to stabilize these variations and fluctuation characteristics is made every day, but is still insufficient. Particularly in a color image forming apparatus, color reproduction is effected with developers (toners) of four colors, i.e., yellow, magenta, cyan and black, superimposed one upon another and therefore, unless the density of developed images, i.e., toner images, of the four colors is accurately adjusted, good color balance cannot be obtained.
Accordingly, in many color image forming apparatuses, there is mounted an image density adjusting mechanism for automatically adjusting image forming conditions such as charging potential, exposure amount and developing bias. A popular method for the image density adjustment is as follows.
First, toner images are formed on an image bearing member or a transfer material bearing member under predetermined image forming conditions, and the density of the toner images is detected by an optical sensor (density sensor) comprising a light emitting element and a light receiving element. Then the image forming conditions are adjusted in conformity with the detected density of the toner images.
In that case, it is known that if density detection is effected by using a regular reflection type sensor great in the light receiving amount and excellent in sensitivity for a black toner, and using a diffuse reflection (irregular reflection) type sensor high in the detection accuracy of high density for the toners of the other colors, i.e., yellow, magenta and cyan, the performance of density control is good, and the method is adopted in many color image forming apparatuses.
Mentioning an example, a toner density detecting apparatus according to Japanese Patent Application Laid-Open No. 6-66722 is an apparatus for applying the light of a light emitting element to an image bearing member on which toner images are formed, and detecting the reflected light thereof by a light receiving element to thereby detect the density of the toners on the image bearing member, and adopts a construction in which a light receiving element for black is disposed at a position for detecting regular reflection light of the reflected light and a light receiving element for colors are disposed at a position for detecting irregular reflection light of the reflected light.
When use is made of the optical type density detecting means, i.e., density sensor, as described above, density detection accuracy is aggravated by the influence of a fluctuation in the quantity of light of the light emitting element and a fluctuation in the light receiving characteristic of the light receiving element, or the unevenness of the attached position of the density sensor, and further a fluctuation in the surface characteristic of the image bearing member or the transfer material bearing member for forming the toner images to be detected, etc. and therefore, correction need be effected by some method.
In the case of the popular regular reflection type density sensor, there is known a method of normalizing the read value of a toner pattern by the density sensor by the detection value (background output value) when the background of an image bearing member or a transfer material bearing member on which the toner pattern is formed is detected by the sensor.
On the other hand, in the case of the diffuse reflection type density sensor, unless the image bearing member or the transfer material bearing member which is the background is of other color than black and the surface characteristic (reflectance) thereof is stable at a predetermined value, the normalizing correction as described above cannot be effected and therefore, the correction of the output value has been difficult. Accordingly, regarding the correction of the diffuse reflection type density sensor, other method is used.
As a conventional example of the correction of the diffuse reflection type sensor, there is a method described in Japanese Patent Application Laid-Open No. 9-284556. An image forming apparatus described in the publication has latent image forming means for forming the latent image of a test pattern on a photosensitive member, developing means for visualizing the latent image, an intermediate transfer member to which the visualized test pattern is transferred, a density sensor for detecting the density of the test pattern, and a reference calibration member in the vicinity of the intermediate transfer member, and is designed to detect the quantity of reflection light of the reference calibration member by a density sensor, and effect gray level correction on the basis of the output value of the density sensor at this time.
As another conventional example, there is a method according to Japanese Patent Application Laid-Open No. 12-258966. The image forming apparatus of the publication is designed to form a pattern for density detection on an image bearing member, a transfer material bearing member or an intermediate transfer member, and in detecting the density of the pattern for density detection by diffuse reflection type and regular reflection type density detecting sensors, normalize the value when the density of the pattern for density detection is detected by the diffuse reflection type density detecting sensor on the basis of the value when the surface of the image bearing member, the transfer material bearing member or the intermediate transfer member is detected by the regular reflection type density detecting sensor.
However, the image forming apparatuses using the correcting method of the diffuse reflection type density sensor as described above have suffered from the following inconveniences.
In the case of the image forming apparatus according to Japanese Patent Application Laid-Open No. 9-284556, it is necessary to newly provide the reference calibration member, and this has resulted in an increase in the number of parts and an increase in costs and the bulkiness of the apparatus. Further, there has been the problem that when the unevenness of the reference calibration member is great, correction accuracy becomes bad, that is, the unevenness of density is made great.
Also, the image forming apparatus according to Japanese Patent Application Laid-Open No. 12-258966 has been effective to correct the unevenness of the quantity of light of the light emitting element, but has been in appropriate for correcting an output fluctuation caused by a reduction in the reflectance of the background and the positional deviation of the sensors. The reason for this will hereinafter be described briefly.
Description will first be made of a case where the quantity of light of the light emitting element of the density sensor has fluctuated. In this case, the light reception outputs of regular reflection light and diffuse reflection light fluctuate at the same rate. Accordingly, if the variable ratio of the regular reflection output value to the background is detected, it is possible to correct the diffuse reflection output by the use of the variable ratio.
On the other hand, when the gloss (reflectance) of the image bearing member or the transfer material bearing member fluctuates, there is a fluctuation about the regular reflection output, but no variation occurs to the diffuse reflection output. Accordingly, if the diffuse reflection output is corrected by the use of the fluctuation rate of the regular reflection output with respect to the background, unnecessary correction is added to the diffuse reflection output which is originally free of fluctuation and all the more, detection accuracy is aggravated. Also, when the positional deviation of the sensors occurs, the regular reflection output strong in directionality fluctuates, but the diffuse reflection output value weak in directionality scarcely fluctuates and therefore, a similar inconvenience occurs.
As described above, the method of diffuse reflection correction described in Japanese Patent Application Laid-Open No. 12-258966 is appropriate in some case and inappropriate in some case in conformity with the fluctuation factors of the output. However, it is very difficult to specify the factors (the fluctuation of the quantity of emitted light, the fluctuation of the background and the positional deviation of the sensors) by which the sensor outputs have been fluctuated. Therefore, when the correcting method is used, the limitation that the fluctuation of the background and the positional deviation of the sensors do not occur becomes necessary. Thus, the method could not be said to be the practically optimum method.
It is an object of the present invention to provide an image forming apparatus which does not require a member such as a reference calibration plate to be newly added and can cope with the fluctuation factors of the output of a density sensor and effect the correction of the sensor output.
It is another object of the present invention to provide an image forming apparatus which is low in cost and excellent in the stability of color reproduction.
It is another object of the present invention to provide an image forming apparatus having an image bearing member or a transfer material bearing member, and density detecting means for detecting the density of an image for density detection formed on the image bearing member or the transfer material bearing member, wherein image forming conditions are controlled on the basis of an output from the density detecting means, the density detecting means has regular reflection type density detecting means for detecting the density by regular reflection light from a portion to be detected, and diffuse reflection type density detecting means for detecting the density by diffuse reflection light from the portion to be detected, and the output value from the diffuse reflection type density detecting means is corrected on the basis of the output value from the regular reflection type density detecting means which has detected a reference image and the output value from the diffuse reflection type density detecting means which has detected the reference image.
It is another object of the present invention to provide an image forming apparatus having an image bearing member or a transfer material bearing member, and density detecting means for detecting the density of an image for density detection formed on the image bearing member or the transfer material bearing member, wherein image forming conditions are controlled on the basis of an output from the density detecting means, the density detecting means has regular reflection type density detecting means for detecting the density by regular reflection light from a portion to be detected, and diffuse reflection type density detecting means for detecting the density by diffuse reflection light from the portion to be detected, and the output value from the diffuse reflection type density detecting means is corrected on the basis of the output value from the regular reflection type density detecting means which has detected a reference image, the output value from the diffuse reflection type density detecting means which has detected the reference image, and an output value obtained by detecting the surface of the image bearing member or the transfer material bearing member by the diffuse reflection type density detecting means.
Further objects of the present invention will become apparent from the following description.